This invention relates to a catalyst for cleaning exhaust gases from internal combustion engines, especially diesel engines. The catalyst comprises a monolithic body having longitudinally extending, open throughchannels coated with a surface-enlarging washcoat layer which is based on silicon carbide and one or more catalytically active precious metals. The invention also concerns a method for preparing the catalyst, the use of the catalyst, as well as an exhaust gas cleaning system and a motor vehicle equipped with the catalyst.
Exhaust gases from diesel engines contain large amounts of soot particles, largely consisting of hydrocarbons, sulphuric acid and water in varying proportions, as well as residues from lubricating oil and engine wear. Usually, the exhaust gases also contain sulphur dioxide. Many systems for cleaning diesel exhaust gases have a particle filter, which increases the pressure drop in the exhaust system. In addition, fairly complicated techniques are needed to avoid clogging of the filter.
Exhaust gas cleaning systems with no particle filter require the presence of a catalyst able to oxidise the hydrocarbon compounds in the particles to carbon dioxide. As is well-known, use can be made of a catalyst in the form of a monolithic body having throughchannels whose walls are coated with a surface-enlarging washcoat layer which is formed of or based on aluminium oxide and is impregnated with precious metals, e.g. platinum and/or palladium. However, such a catalyst also oxidises a substantial amount of the sulphur dioxide to sulphur trioxide, which is more corrosive as well as more harmful to the environment. At low temperatures, sulphur dioxide and sulphur trioxide may, in addition, be absorbed by the catalyst, to be desorbed later on when the temperature of the exhaust gases rises. When the temperature of the catalyst increases rapidly, the amount of sulphur dioxide and sulphur trioxide desorbed may be considerable. Among other things, this manifests itself as an increase in the emission of particles from the catalyst. A further problem arises when inorganic residues of lubricating oil additives, such as compounds of phosphorus, zinc, calcium, magnesium and sulphur, are present in the exhaust gases and are enriched in the catalyst, thereby considerably reducing the catalytic activity in course of time.
EP-B-0 432 534 discloses a monolithic catalyst for cleaning diesel exhaust gases. This catalyst has a surface-enlarging layer containing aluminium oxide, titanium dioxide and silicon dioxide. As catalytically active substance, use is made not only of precious metals, but also of a vanadium compound stated to reduce the oxidation tendencies of SO2 and NO.
PCT Publication WO 90/05579 teaches an oxidation catalyst containing precious metals on a carrier of silicon dioxide, titanium dioxide, zirconium dioxide or zeolite, stated to reduce the sensitivity of the catalyst to sulphur pollutants in the exhaust gases from fossil fuels.
Further, it is known to prepare porous catalyst bodies by sintering silicon carbide into monoliths. This technique is described e.g. in Derwent Abstracts WPI Acc. No. 90257450/34, JP Kokai 2-180641. The purpose is to speed up the increase of the catalyst temperature up to running temperature. It is stated that a surface layer of silicon dioxide of sustained strength can be produced by treatment under oxidising conditions. This catalyst has no surface-enlarging washcoat layer.
U.S. Pat. No. 4,812,434 discloses a homogeneous catalyst consisting of 50-95% by weight of SiC and 5-50% by weight of an alloy made up of silicon and one or more of the following metals: Cu, Fe, Co, Ni, Zr, V, Mo, Mn, Zn, Ag, Pt and Pd. The alloy acts as catalytically active material and is activated by oxidation or chemical treatment. This monolith has been prepared from the indicated material for improved thermal stability and increased chemical resistance and mechanical strength. Further, the fact that the catalytic material is evenly distributed over the whole cross-section of the catalyst is seen as an advantage. Thus, the U.S. specification relates to a catalyst having improved mechanical and thermal properties, being more available in terms of economy, and/or having improved long-term strength in operation.
There is a need for an effective oxidation catalyst which is suitable for cleaning diesel exhaust gases and is comparatively passive in respect of oxidation and accumulation (absorption) of sulphur dioxide, and which is resistant to accumulation of inorganic residues of lubricating oil additives, such as contaminants of phosphorus, zinc, calcium, magnesium and sulphur, especially to enrichment of phosphorus. Also, the absorption of sulphur dioxide and sulphur trioxide in the catalyst should be minimised.
As mentioned earlier, exhaust gas cleaning involves a considerable problem residing in the oxidation, in prior-art catalysts, of sulphur dioxide to sulphur trioxide. There is also the problem of inorganic residues of lubricating oil additives accumulating in the catalysts. One object of the present invention is, therefore, to obviate these problems in connection with a catalyst whose carrier is a monolithic body having longitudinally extending, open throughchannels which have a surface-enlarging washcoat layer impregnated with the active catalyst material. According to the invention, this object is achieved by the washcoat layer being formed of a substantially aluminium-oxide-free layer of finely distributed silicon carbide and impregnated with an effective amount of one or more catalytically active precious metals. Preferably, the content of silicon carbide in the washcoat layer is at least 90% by weight.
The characteristic features of the invention are recited in appended claim 1, and especially preferred embodiments are defined in the appended subclaims.